1. Field of the Invention
The present invention relates generally to bearing arrangements.
2. Description of Related Art
Generally speaking, bearings are intended to allow relatively free operational movement of a pair of components, whilst transmitting forces between the components thereby locally to put a relative kinematic constraint on at least one degree of freedom of the components. For example, a rotary bearing will allow relative rotation of components but will transmit force in one or both of a radial direction and a thrust direction to provide a local kinematic constraint in these directions.
By “kinematic constraint” is meant prevention or arrest of motion of the components.
Bearings typically fall into two classes: rolling element bearings or plain bearings.
In the case of rolling element bearings, these may take the form of conventional ball bearings or roller bearings.
In the case of plain bearings, these may generally take the form of lubricated “metal-on-metal” plain bearings, dry or “self-lubricating” plain bearings comprising a non-metallic bearing liner (typically a polymer liner), or fluid bearings.
A problem with rolling element bearings and “metal-on-metal” plain bearings is that they tend to be “stiff” in the load-bearing direction and therefore provide a very rigid kinematic constraint. This stiffness may be disadvantageous in certain applications, where a certain degree of compliance in the bearing arrangement can help to limit contact pressures, which would otherwise be high because of the small contact area of the rolling elements, and therefore reduce bearing wear caused by manufacturing tolerances in the bearing, misalignment of separate bearings, as well as vibration and/or residual static and dynamic shaft imbalance in the case of rotating components. Additionally rolling element bearings have the disadvantage that they require a supply of lubrication.
The problem of wear can be significantly reduced by using “low friction” hydrostatic or hydrodynamic fluid bearings. However, these bearings require a supply of pressurised fluid in order to maintain their function (which may need to be supplied at a particularly high pressure in the case of “non-contact” hydrostatic bearings) and this generally requires relatively expensive, complicated and bulky ancillary equipment.
A “metal on metal” plain bearing may suffer high contact load in a contact zone between the bearing element and liner that may threaten the integrity of the bearing material. Plain bearings utilising a polymer liner which spreads the contact load over a greater area, thereby reducing the contact pressure, offer significantly increased compliance as compared to conventional rolling element and “metal on metal” plain bearings and can therefore provide a compliant kinematic constraint which may be effective in limiting contact pressures at the bearing surface. At the same time, the bearing function is not dependent upon an ancillary supply of working fluid as in a fluid bearing and therefore such bearings may offer a relatively simple and low-cost overall design, particularly where a dry “self-lubricating” polymer liner is used that does not require a separate supply of lubricant. However, polymer lined bearings tend not to be suitable for high temperature applications because at such temperatures significant degradation of the polymer liner may occur, compromising bearing performance.
Rolling element, hydrodynamic and hydrostatic bearings must be accurately aligned to operate effectively. The accuracy required of the bearing unit and location features on the device to which it is mounted increases the complexity of manufacture and hence the cost of the final product.